(1) Field of the Invention:
This invention relates to a variable equalizer. More particularly, it relates to a variable equalizer which can vary the frequency characteristic of a transmission line for communication and which is incorporated into the transmission line in order to compensate for a change in the frequency characteristic.
(2) Description of the Prior Art:
In a wire communication system employing a coaxial cable, a pair cable or the like, the transmission characteristic of the cable fluctuates in dependence on the length of the transmission distance, a change in the ambient temperature, etc. It is therefore necessary to compensate for the fluctuations with a repeater, a receiver or the like.
Types of equalizers for making these compensations includes frequency equalizers which are determined by the characteristic inherent to a transmission line, and equalizers which compensate for the fluctuations of the characteristic inherent to the line as they occur due to changes in the ambient conditions of temperature etc.
The characteristics, i.e., gains of such equalizers for compensating for the characteristic fluctuations due to the changes of the ambient conditions are represented, when broadly classified, by a function of (XY+1)/(X+Y) and a function of (X-Y)/(X+Y). Here, X denotes a variable resistance value, and Y denotes an impedance which fluctuates in dependence on the frequency characteristic. In the former equalizer, the variable range of the variable resistance must cover from zero to infinity, which is inconvenient in constructing a circuit device. In contrast, in the latter equalizer, the variable range of the variable resistance may be from -R to +R. Such an equalizer has the advantage that the variable resistance can be constructed of a field-effect transistor (FET) or similar device.
As an equalizer belonging to the latter type, there has heretofore been proposed an effective variable equalizer which employs a single variable resistance and a single operational amplifier, and which employs neither an inductor nor a negative resistance (see IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS, VOL. CAS-22, No. 8, AUGUST 1975, "Minimally Active RC Variable Equalizers," FRANC BRGLEZ).
The known equalizer, however, is so constructed that two circuits (on the gain side and on the loss side) are changed-over by a switch in order to vary the frequency characteristic symmetrically with respect to a reference characteristic. This leads to the disadvantage that the construction and use of a device is inconvenient.